Sound recording system



* y 1944. F. G. ALBIN SOUND RECORDING SYSTEM Filed June 20, 1942 2 Sheets-Sheet l m. wmm m m 1 if P m A W r 7/ 8 .e G MMM ii esoae/cz G.Az.a/-,

INVENTOR ATTO R N EY Julyv 1944- I F G. ALBlN 2,354,295

SOUND RECORDING SYSTEM Filed June 20, 1942 2 Sheets-Sheet 2 Q W; 12 W0 4';

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. INVENTOR ATTORNEY Patent ed July 1944 UNITED STATES PATENT 7 OFFICE SOUND RECiZSfZG SYSTEM I Frederick c. Albin, Los Angeles, Calif., asslgnor to-Badio-Oorporation of America, a corporation of Delaware Application June 20, 1942, Serial No. 447,197 g Y .5 Claims. (01. 179-1003) Two'standard types of commercial sound onfilm recording systems are well known, the variable area system producing a longitudinal traceor'traces along the film to separate opaque and transparent areas, and the variable density system producing a series of transverse striations of variable density. The usual manner ofrecording the variable area type of sound record is with a galvanometer having a light reflecting mirror which deflects a beam of light in accordance with the instantaneous amplitudes ofelectrical curon the galvanometer mirror, the shaped beam being reflected to a mask having a slit through which a certain amount of light passes in accordance with the amplitude of'the signal.

Although it is well known that a galvanometer may be used as a modulator for producing varithrough which is inaccordance with the instantaneous values of the signal currents, one ribbon always moving in the direction of film motion when the other ribbon is moving in the opposite direction.

The galvanometer, as a modulator, has certain advantages overthelight valve and vice versa,

the present invention being directed to asystem which is adapted to utilize the best features'of both systems. For instance, the present invention employs a-galvanometer, which is the preferred modulator, the galvanometer being used in an optical system to simulate the desirable action of adouble ribbon light valve. This eliminates the light valve, which is delicateeasily damaged by overloads and which'requires constant checking and adjustment: The galvanometer on the other hand, is rugged and reliable and may operate for weeks and months without attention or adjustment.

In the light valve type of system,-the light valve ribbons form a narrow slit which is imaged on the film by a light of constant intensity and uniform distribution over the area of the slit.

When-modulated by a signal, the spacing of the ribbons vary, resulting in avariation in width of the image A' point on the fil'm' passing through the image is exposed for a longer or shorter time period, depending on the distance through the image corresponding to the image width. Thus, the film exposure is a time func;

tion of constant intensity light and the-maxi;

mum peak exposure is limited only by the maximum allowable instantaneous valve spacing which may exceed the average by many fold. Inherent in this type of system is a serious distortion of wave form and level as a function of rents being recorded. Such variable area systems generally shape the light beam before impression closed hereinafter.

mean image width and velocity of change in width relative to film velocity. Consequently, the mean slit width is operated at a low value except for brief intervals when quality may be sacriflced for higher levels to gain dramatic effect.

On the other hand, the galvanometer type of system is one Ofvariablelight intensity wherein the slit image width on the film is constant and the distortion inherent in the variable. slit width system .due to velocity eflects is avoided. With fixed slit image width and constant light intensity, the maximum possible exposure is established at a certain value by the limitation of the light intensity and the choice of a slit image 'width determined by the optimum resolution of tem when light intensities and mean slit sizes for thetwo systems are comparable.

A further feature of. the variable slit system is J in providing the maximum of high frequency resolution at low levels, thereby improving articulation and intelligibility and providing attenuation of the high frequencies at high levels when they are not required for intelligibility and their presence is irritating and generally unpleasant.

The present invention obtains the desirable variable time feature of the light valve by the use of the favored galvanometer structure in the recording of variable density-records. It may be used in the production of both a standard type of record and records of the push-pull type, while noise reduction may be incorporated in the. system in any standard manner, such as will be dis- The principal object of the invention; therefore, is to facilitate the recording of photographic sound records.

56 A further object of the invention is Another object of the invention is to facilitate and simplify the recording of variable density sound records of high intelligibility.

to provide a system of recording variable density sound records having high intelligibility.

A further object of the invention is to provide a variable density recording system employing a galvanometer as the modulating element.

A further object of the invention is to produce a sound recording light beam in which the dimensions thereof longitudinally of the film vary in opposite directions during a single directional movement of a galvanometer mirror.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appended herewith, the manner of its organization and the tlon of the mirror 29 in vibrating the light beams is the same as though the mask l6 were moved up 'and down in a vertical direction in front of the mask 2|. To illustrate. the upper beam passing through aperture H and aperture mode of its operation will be better understood by 22 will be called the A beam, and the beam passreferring to the following description read in conjunction with the accompanying drawings, forming a part hereof, in which:

Fig. 1 is a combination block and diagrammatic ing through apertures l8 and 23 will be called the B beam. Assuming for the moment that noise reduction is not employed, the normal adjustment of the galvanometer |2 for no signal would view of the film sound recording system embodybe such that half of the possible total width of ing the invention;

Fig. 2 is a detailed view of the optical portion.

of the invention showing the path of the principal light rays;

, the light beams A and B are trrnsmitted to the optical elements 33, 34 and 35. With the mask |6 moved upwardly so that slits l1 and I8 are in optical alignment with slits 22 and 23, the full Figs. 3 and 4 are detailed views of a cross- -35 widths of the beams are obtained'corresponding Fig. 5 is a view of a modification of a. portion of the optical path of the invention;

to 100% transmission of light, while an .equal downward movement of mask IE will substantially eliminate the beams from the film.

Thus, theflillustration of the beams A and B Fig. 6 is a view of another modification ofthe as they pass through the elements 23, 24 and 25 v optical portion of the system showing the paths of the extreme rays; and, s

Fig. '7 is a detailed view of a modified mask for producing a push-pull type of record.

I th? I directly through the element 33 with only a slight changein position while the beam A is inverted isfo a no signal condition without noise reduc- It is to be noted that the beam B passes Referring now to Fig. 1, the electrical as-itypasses-gthrough the prism 34. The element of the recording system includes a microphone 5 connected to an amplifier 6, the output of which 33 is usedfor the purpose of equalizing the light intensity and providing equal path lengths for is varied by a mixer I before beingfed into an bothoi the beams. After inversion and equalizaamplifier 8. The output of the'amplifier 3 is tion of the beams, the parallelepiped brings divided, one portion going to a noise reduction thebeams together and reverses their position amplifier l0 and another portion being impressed upon a galvanometer I2 over conductor l3. The output of the noise reduction amplifier H! is iin'. pressed over conductors H on a solenoid adapted so that the A and B beams as projected upon the film are now in the same relative position with respect to each other as they were at slits 22 and 23, except that the A beam has been inverted.

to move a mask Hi, the mask having rectangular 5 It will be noted that the fixed side A of beam A apertures l1 and I8 therein. As shown by the dotted lines 20, the output of the noise reduction amplifier may either be connected to the solenoid for operating the mask It or to a, solenoid for at slit 22 and fixed side B of beam B at slit 23 coincide on the film 39 at the optical axis C.

A cross-section of the beams as projected upon the film is illustrated in Fig. 3, wherein the similarly operating a mask 2| having rectangular dimension a represents the full possible width of slit apertures 22 and 23 similar to apertures l1 and H! of mask it. The dotted line portion may be substituted for the full line portion. so that the noise reduction may be obtained by either the two beams A and B, while the dimension D is the actual width at no signal as illustrated in Fig. 2. Beam A is the upper half and beam B the lower half, the line 0 being the center or axis of operating mask IS with mask 2| fixed,'-"or by the combined beams and the line A'B' of coinoperating mask 2| with mask 13 fixed.

Referring now to the optical portion of. the

recording system, a light source "25 produces a light beam which .is collected by a. lens 26 and .cidence.; As the galvanometer mirror vibrates, the beams A and B will increase and decrease in width, the result being a single recording beam inwhich the width varies equally from the axial projected onto mask IS, the light emrging no line C. Thus. one direction of motion of the mirthrough apertures l1 and I! being projected on a a mirror 29 of galvanometer l2 by a lens 28. The lenses 26 and 28 image the filament of lamp 25 on the mirror 29 of galvanometer |2.. The light ,ror 29 causes the upper and lower boundaries of the beam or beams to separate, while the reverse motion of the mirror causes these boundaries to approach each other. This variation of reflected from th m -01- 29 i projected by a the combined beams is the same as that of the lens 3| onto the mask 2|. the lenses 28 and 3| imaging the apertures l1 and I8 on the slit mask 2|. The light emerging from the slit 23 is projected by a lens 32 to a rectangular optical unit 33 having zero'optical power. emerging from the slit 22 is projected by'lens 32 to a triangular prism element 34 which ele- 'ment inverts the light beam before passing it to a rectangular parallelepiped 35. The light from image produced by the light valve ribbons. For low amplitude signals, therefore, the beams will be narrow and for signals of higher amplitude, the beams will be wider, but regardless of width The light of the full recording beam, the long sides thereof will be equally spaced from the center axial line C.

Although this action is the same-as that of a light valve; the galvanometer produces a more the element 33 also strikes the element 35 and accurate action than that of the light valve. In

the present system the increase and decrease in width of the complete beam are each produced by the movement of a single element in a single direction and the outer edges of the beams must therefore move the same distance. In a light valve, each edge of the beam is controlled by a different ribbon which, if not identical in size and tension, will move differently even when conducting the same current. Thus, the galvanometer is not only more rugged than the light valve, but produces a more uniform widening and narrowing of the recording beam about a central axis.

, For the application of noise reduction to the system just described, it is only necessary, as mentioned above to control either mask I6 or mask 2| with rectified currents corresponding to the average amplitude of the signal currents. When impressed upon the mask l6 or upon the mask 2|, theadjustment is suchas to permit a narrow beam oflight to strike the film during times of no signal, such a narrow strip of light being shown by the dark portion of Fig. 4. The adjustment may be accomplished by either me.- chanically or electrically biasing the mask 16 'or mask 2| to this no signal position. Now, as the-amplitude of the signal increases, the mask 56 will be moved with respect to mask 2| or vice versa, so that for 100% modulation the average width of the beam is as shown in Figs. 2 and 3. Thus, the application of noise reduction produces a final print which is highly opaque during times of no signal or low modulations and has an average exposure of 50% during 100% modulation. The invention has so far been described as a system using three separate optical elements 33, 34 and 35, which if so used would be difiicult to adjust and maintain in their proper optical relationship with one another and with the remaining elements of the optical system. Thus, as shown in Fig. 5, a two-element arrangement is provided wherein an upper parallelepiped element 40 transmits beam A and a trapezoidal optical element 4| inverts and transmits beam B. These optical elements are so arranged that the beams A and B do not cross as in Fig. 2, but maintain their same relative posi- To illustrate the light transmission through element 43 as well as the elements in Figs. 2 and 5, the extreme rays have been traced through the element 43 from fixed points A and B' on slits '22 and 23. From the point A at the lower side of slit 22, rays D and E form the virtual image thereof at point DE in the lower section of element 43 while a virtual image FG of point B on the lower-side of slit 23 is formed by extreme rays F and G-in the upper section of 43. The rays D, E, F, and G are refracted through element 43 as illustrated, to form the axial line A'B' along the optical axis C of Figs. 3 and 4.

As mentioned above, the system is suitable for the recording of a push-pull type of sound record, such recording being accomplished by substituting the two-mask elements 45 and 46 of Fig. 'l for the mask l6 of Fig. 1. In this mask modification, the mask 45 has a.v pair of apertures 48 and 49 while the mask 46 has a similar pair of apertures 5| and 52. To provide a push-pull record, however, the sections of the mask unit 45-46 are moved in opposite directions by a link connection 53, 54 and 55, the link 54 being pivoted at 56 to provide the reversal of movement of the apertures 48-49 with respect to apertures 5|-52. It will be observed that in this arrangement when the beams through apertures 5i and 52 are being tions with respectto one another on the entrant and emergent sides of the elements. The same operation is obtained, however, with elements 40 and 4| as with elements 33, 34 and 35; that is, the outer edges of the combined beams move away from and toward one another with variations in amplitude of the signal. In this arrangement objective lens 42 may be used intermediate the film 39 and elements 48 and 4!. To further decrease the number of optical elements, reference is made to Fig. 6, which shows a single optical unit 43 which performs in the same manner as the three optical elements 33', 34 and 35 in Fig. 2. Beam B of Fig. 2 strikes the upper left-hand surface of the element 43 and is deflected downwardly to be refracted from the lower right-hand surface of the element 43. Beam A of Fig. 2 strikes a similar surface. as the surface struck by beam B but is inverted by the lower surface of the element 43 and reflected upwardly to cross beam A and be reflected by the upper right-hand surface of the element 43. The beams A and B, therefore, are reversed as to position in the same manner as in Fig. 2, while he beam Ais inverted in the same manner as in Figs. 2 and 5 by the use of a single optical element. The problem of adjustment of several optical elements is thus avoided.

I-claim as my invention:

increased in width upon half of the film track, the beams through apertures 48 and 49 are being decreased in width as they strike the other half of the film track.- This opposite and equal reaction produces the well known type of push-pull sound record, each half operating to produce a record of the same typeas the record just described.-

Also, each half of the track will have all the advantages of the single track described in connection with the Figs. 1 to 6, inclusive.

The above-described system, therefore, is a variable density sound recording system which utilizes a galvanometer to produce a variable time light beam varying in width upon a film. vIn addition to these advantages attendant upon the use of this particular type of expanding and contract ing light beam, the system also has the advantages attendant upon the use of a galvanometer as the light modulating instrumentality.

1. A variable density sound recording system comprising means for generating electrical currents corresponding to sound waves, means for forming a plurality of light beams, a moving film to which said beams are adapted to be projected, means for simultaneously varying the width of said beams in the direction of the motion of said film in accordance with the instantaneous values of said electrical currents, one side of each of said beams remaining fixed, means for inverting one of said beams, and means for combining said beams into a single beam, the fixed sides of said beams being coincident at the center axis of said single beam.

2. A variable density sound recording system in accordance with claim 1 in which said beam forming means is adapted to vary the average width of said beams in accordance with the average amplitude variations of said electricalcurrents.

3. A sound recording system in accordance with claim 1 in which said last mentioned means includes at least one optical element for inverting one of said beams and deflecting said plurality of beams to form said single beam.

4. The method of recording a variable density record comprising generating electrical currents corresponding to sound waves, generating a plurality of light beams, vibrating said plurality of light beams in accordance with the instantaneous values of said electrical currents, said vibration increasing and decreasing the width of said beams in the direction of motion of a record medium along one side only of each of said beams, and deflecting said beams to form a single beam adapted to vary in width uniformly in both directions about a fixed central axis in accordance with the instantaneous value of said electrical currents.

5. The method of claim 4 in which the average width of said plurality of beams and said single beam is varied in accordance with the average value of said electrical currents.

FREDERICK G. ALBI-N. 

